1. Field of the Invention
The present invention relates to methods for deinking and decolorizing printed paper.
2. Description of the Related Art
Printed waste paper is an important source for producing recycled paper. Recycling of printed waste paper generally requires deinking and bleaching of the printed waste paper to a suitable brightness before being used in the production of recycled paper.
Current industrial deinking/pulp brightening practices are schematically shown in FIG. 1. The waste paper is first repulped at moderate to high consistency (6-20%) in water followed by deinking of the pulp. The deinking step is essentially a separation method employing chemicals, heat, and mechanical energy to dislodge ink or toner particles from the fibers. The traditional approach to deinking is to break down the ink particles to a fine size using mechanical action in the presence of conventional chemicals, e.g., caustics, surfactants, and hydrogen peroxide. The chemical systems mainly used in deinking waste papers employ an alkaline system usually with hydrogen peroxide to maintain pulp brightness and impede alkaline "darkening" of those pulp fibers which originate from groundwood containing papers. The surfactants used in the alkaline deinking systems are generally synthetic detergent formulations or fatty acid soaps. In the latter case, additional calcium may be required to enhance the action of these materials, but which can also form unwanted deposits around the deinking process. After sufficient pulping time to achieve dislodging of the ink particles from the fiber surface, the pulp slurry is diluted to a lower pulp consistency, and held for a period in a dump chest.
The pulp is then usually further diluted and other contrary materials removed via centrifugal cleaning and screening. At this point, the pulp is directed to the flotation stage where air is introduced into the pulp slurry. The ink particles adsorb to the air bubbles and are carried to the top of the pulp slurry forming a foam due to the presence of the surfactant or soap preparations. The foam is then separated from the rest of the mixture by mechanical means. One newer technology employs chemical-mechanical deinking based on densification chemistry to increase the size of the dislodged ink particles which then can be removed by centrifugal cleaning and screening.
After flotation, the pulp is thickened and stored for papermaking use, or is post-treated to improve the brightness of the pulp prior to papermaking. The post-treatment is separate from the pulping/deinking process. For those papers contained in the waste stream to which some form of coloration was applied, dye-stripping of the pulp typically involves chlorine-based bleaching or chlorine-free bleaching.
Chlorine-based bleaching routinely uses elemental chlorine or sodium hypochlorite bleaching sequences which produce the maximum contaminant destruction, color removal, and brightness increase. However, hypochlorite is not effective with furnishes containing more than about 10% mechanical pulp. Furthermore, chlorine-based bleaching sequences are environmentally unfriendly. Chlorine-free bleaching uses oxygen, ozone, hydrogen peroxide, sodium hydrosulfite (reductive), or formamidine sulfinic acid (reductive). However, these bleaching agents either provide only moderate improved brightness of the pulp, are expensive, are ineffective, or can adversely affect the properties of the pulp.
The object of the present invention is to provide improved methods for deinking and decolorizing printed paper.